Abstract : The impact of assimilating Zenith Total delay (ZTD) observations from a mesoscale ground-based GPS network over Western Europe is evaluated for the heavy precipitation event of 5–9 September 2005 over Southern France. The ZTD assimilation is performed using a three dimensional variational data assimilation system at the 9.5-km horizontal resolution. Then using as initial conditions the 3DVAR analyses with and without assimilation of ZTD, we perform 2.4-km non-hydrostatic MESO-NH simulations. The results of the fine-scale simulations indicate that assimilation of ZTD help to improve the forecast of the tropospheric water vapour content and the quantitative precipitation forecast. We have also assessed through single observation experiments the influence of the formulation of the observation operator which is used to compute the model equivalent ZTD.